In the manufacturing of web materials, large rolls of the material are produced. These large rolls are subsequently processed to produce a finished product. The conversion of the roll to a finished or intermediate product requires the transport and unwinding of the roll of web material.
Web-converting processes include a roll unwinding apparatus configured to unwind a horizontally oriented roll to present the web to the converting equipment in a horizontal orientation. A horizontal roll may be core driven; it may be compressed along the longitudinal axis and driven on the end surfaces of roll. The roll may also be driven using belts in contact with the outer surface of the roll. Low-density rolls may be adversely affected by being surface driven. For example, a 250 cm. diameter roll that is 255 cm wide and weighs 1600 kg, may be supported by 5 belts each 15 cm. wide over a circumference arc of 100 cm. This drive produces a compressive force in the supported areas of 20,700 N/m2. These compressive forces can alter the tissue web's unwinding speed, distort the webs, and lower the quality of the finished products made from the webs.
Horizontal rolls may acquire an egg-shaped cross section rather than the desired round cross section. 15 to 20 cm. eccentricity is common in rolls having a diameter of 250 cm. Unwinding an egg shaped roll is problematic in that the mass of the roll is not balanced about the longitudinal axis. This imbalance results in additional strain on the unwinding mechanism as the forces generated by the rotating roll fluctuate with the unbalanced mass. These forces are directly proportional to the degree of imbalance present in the roll and the speed of rotation of the roll. Severely unbalanced rolls must therefore be unwound slowly to avoid subjecting the unwinding apparatus to destructive forces. Furthermore, the unwinding of the unbalanced roll can cause the speed and tension of the web to fluctuate considerably. These speed and tension fluctuations can result in web breaks and lost production time. Again the affect of the unbalanced roll is more severe at higher speeds so again the unwind speed must be slowed to reduce the incidence of web breaks. The rate at which an unbalanced roll may be reliably unwound limits the rate of the downstream process. The fluctuations in web speed and tension can affect the quality and uniformity of the converted product.
The fluctuations in the web speed and tension also impair the ability of the web processor to splice multiple rolls of material without stopping the unwinding process or without extensive capital investment in splicing equipment to enable a flying splice despite the fluctuations in tension and speed. Splicing methods known in the art require the webs to have matched speeds at the time of splicing. The inability to maintain a consistent web speed thus requires stopping the web and in some instances the entire process to splice rolls together resulting in lost production time.
After a stoppage, the production equipment must be accelerated back to production speeds during which time more productivity is lost. Then the spliced portion of the web must be removed from the finished product. Due to the fluctuations in speed before and after the splice it is often necessary to remove a substantial amount of product to ensure that the spliced portion is removed. This results in high material losses.
This invention provides a method and apparatus for unwinding a roll of a web material that will enable high speed unwinding of the web while maintaining narrow limits on the fluctuations in the speed and tension of the web.
This invention further provides a method and apparatus for unwinding a web that includes a reliable means of splicing multiple webs without stopping the unwinding process.